African Wildlife & Environment Issue 83

CONSERVATION

IS THE UMZIMKHULU ESTUARY LIKE THE OKAVANGO DELTA?

In a previous article, published in African Wildlife and Environment , I mentioned that the uMzimkhulu is similar in ecological terms to the Okavango The uMzimkhulu is one of the last of our free-flowing rivers in South Africa where a natural flood pulse interacts with the tidal pulse to sustain one of the last fully functional estuarine ecosystems on the Indian Ocean coastline of South Africa.

Dr Anthony Turton

In the case of the Okavango, there are two main rivers – Cuito and Kavango – each with a slightly different flood pulse, but both combining to create

Delta, simply by virtue of the biodiversity both sustain from the unique flood pulse in each system. This begs the question as to how these two rivers might be different. To answer this question, the reader first needs to understand what a natural flood pulse is, to grasp what the impact would be from an altered flood pulse. We can think of a flood pulse as the ‘natural heartbeat’ of a river. The flow in any river is never constant. Instead, it pulses as a direct result of rainfall events in the headwater catchments of the river basin.The peak of the pulse determines the extent of flooding, so it’s highly relevant to wetlands and nutrient distribution, while the ebb of the pulse determines the survival of species trapped in pools during the low flow periods. It’s the difference between these

The Okavango Delta showing tectonic faults

the annual flooding of the Delta. If that flooding is big, then the resultant flow eventually enters the Boteti River that flows into the Makgadikgadi Salt Pans. The Delta is formed by tectonic activity, with two major fault lines defining both the proximal and distil end of the system. It is possible that the faulting might be the southern extremity of the Great Rift Valley of Africa, but some geologists debate this. To make matters even more complex, the crust between these two fault lines is tilting over long time scales, which is a factor that drives the distribution of water and silt into the various distributaries that each terminate along the Kunyere and Thamalakane fault lines. However, the same fault that has captured

two extremes that determine how the downstream ecosystems function. It therefore also determines what the species distribution is over time. This flood pulse is attenuated by dams built on any river, so the impact is disproportionally large because it's amplified over so many different factors like habitat, refugia for key species, and the ultimate diversity of the biomass within each sub-system. Given that both the Okavango and the uMzimkhulu have no dams built on them, they both have a fully functioning natural flood pulse. That flood pulse is complex, because it links the headwaters with the receiving ecosystem, each of which typically has a unique set of characteristic features.

13 | African Wildlife & Environment | Issue 83 (2023)